1. Field of the Invention
The present invention relates to hydraulic drive controls for production machines and, more particularly to a hydraulic control circuit which makes it possible to drive two loads in an injection molding machine, for example, by means of a hydraulic pump, in accordance with separate flow rate programs and pressure programs for the two load lines supplying drive fluid to the two loads.
2. Description of the Prior Art
One of the primary objectives in the design of hydraulic drive systems is the maximization of the hydraulic efficiency of the drive system, i.e. the goal of avoiding, as much as possible, the conversion of hydraulic drive energy into heat in connection with the use of fluid throttling devices.
It has therefore already been suggested to drive a hydraulic load--typically a cylinder assembly or a hydraulic motor--at changing speeds and force or torque, with the aid of a variable-delivery pump, preferably a pilot-operated variable-delivery pump. In such a hydraulic control circuit, the output setting of the variable delivery pump is continuously adjusted by means of a pump adjusting valve in a feedback loop, the pump adjusting valve controlling the pump output so as to maintain a small constant operational pressure gradient for a flow control valve in the load line, thereby minimizing the throttling action of the flow control valve.
The use of a computer for the storage and generation of control input data, with its advantages of simplicity and unlimited adaptability to changing operational requirements through digitization and storage of the control information, has brought with it a need for hydraulic control valves which are capable of translating continuously changing electronic input signals into correspondingly changing hydraulic values of flow rate and pressure in a load line.
Best suited for this purpose are electromagnetically driven valves of the type which are known as proportional-response valves. A valve of this type has its valve spool connected to a special proportional-response solenoid of which the armature moves to, and is retained in, an axial position which is proportional to the value of the input signal being fed to the control circuit of the proportional-response solenoid.
Included among the known proportional-response valves are proportional flow control valves, proportional pressure control valves and the more recently proposed proportional pressure and flow control valves, or so-called proportional P/Q valves. A proportional P/Q valve is capable of controlling either the flow (quantity "Q") or the pressure ("P") in a load line, depending on whether speed or force is the governing parameter of the hydraulic control program.
In connection with the hydraulic drive system of an injection molding machine, it has already been suggested to use, as part of an energy-efficient hydraulic control system, a pilot-controlled variable-delivery pump which has arranged in its load line a proportional P/Q valve and a pressure transducer. By means of the proportional P/Q valve, either the flow rate or the pressure in the load line is continuously adjusted in accordance with either a flow program determining the speed of the load, or a pressure program determining the force which is applied to the load.
Through a feedback line which links the downstream side of the proportional P/Q valve to the pump adjusting valve of the variable-delivery pump of the hydraulic system, the pump output is continuously adjusted to maintain an output pressure which exceeds the drive pressure in the load line by the operational pressure gradient. Such a hydraulic control system is disclosed in the German Pat. No. 31 19 095 and in the corresponding Canadian Pat. No. 1,171,757.
This known hydraulic control circuit has the shortcoming that it is not suitable for the simultaneous driving of two loads in a production machine in accordance with separate speed and force programs.